//1、var允许对同一个变量重新定义，把原有变量值改变
var a = 10
var a = 'hello'
console.log('a:',a)


//let允许修改
let b = 100
// let b = 1000
b = 'helloworld'
console.log('b:',b)

//const不允许修改
const c = 500
// c = 1000
